text: ::(For more details on displacement buoyancy-driven ventilation (rather than mixing type buoyancy-driven ventilation), see Stack effect)
  Buoyancy driven ventilation arise due to differences in density of interior and exterior air, which in large part arises from differences in temperature. When there is a temperature difference between two adjoining volumes of air the warmer air will have lower density and be more buoyant thus will rise above the cold air creating an upward air stream. Forced upflow buoyancy driven ventilation in a building takes place in a traditional fireplace. Passive stack ventilators are common in most bathrooms and other type of spaces without direct access to the outdoors.
  In order for a building to be ventilated adequately via buoyancy driven ventilation, the inside and outside temperatures must be different. When the interior is warmer than the exterior, indoor air rises and escapes the building at higher apertures. If there are lower apertures then colder, denser air from the exterior enters the building through them, thereby creating upflow displacement ventilation. However, if there are no lower apertures present, then both in- and out-flow will occur through the high level opening. This is called mixing ventilation. This latter strategy still results in fresh air reaching to low level, since although the incoming cold air will mix with the interior air, it will always be more dense than the bulk interior air and hence fall to the floor. Buoyancy-driven ventilation increases with greater temperature difference, and increased height between the higher and lower apertures in the case of displacement ventilation. When both high and low level openings are present, the neutral plane in a building occurs at the location between the high and low openings at which the internal pressure will be the same as the external pressure (in the absence of wind). Above the neutral plane, the internal air pressure will be positive and air will flow out of any intermediate level apertures created. Below the neutral plane the internal air pressure will be negative and external air will be drawn into the space through any intermediate level apertures. Buoyancy-driven ventilation has several significant benefits: {See Linden, P Annu Rev Fluid Mech, 1999}
  * Does not rely on wind: can take place on still, hot summer days when it is most needed.
  * Stable air flow (compared to wind)
  * Greater control in choosing areas of air intake
  * Sustainable method
  Limitations of buoyancy-driven ventilation:
  * Lower magnitude compared to wind ventilation on the windiest days
  * Relies on temperature differences (inside/outside)
  * Design restrictions (height, location of apertures) and may incur extra costs (ventilator stacks, taller spaces)
  * The quality of air it introduces in buildings may be polluted for example due to proximity to an urban or industrial area (although this can also be a factor in wind-driven ventilation)
  Natural ventilation in buildings can rely mostly on wind pressure differences in windy conditions, but buoyancy effects can a) augment this type of ventilation and b) ensure air flow rates during still days. Buoyancy-driven ventilation can be implemented in ways that air inflow in the building does not rely solely on wind direction. In this respect, it may provide improved air quality in some types of polluted environments such as cities. For example, air can be drawn through the backside or courtyards of buildings avoiding the direct pollution and noise of the street facade. Wind can augment the buoyancy effect, but can also reduce its effect depending on its speed, direction and the design of air inlets and outlets. Therefore, prevailing winds must be taken into account when designing for stack effect ventilation.